1. FIELD OF THE INVENTION
This invention relates to a method for the evaluation of spatial distribution of deep level concentration in a semiconductor crystal and more particularly to improvements in and concerning a method for the evaluation of the spatial distribution of relative deep level concentration near the surface of a semiconductor wafer.
2. DESCRIPTION OF THE PRIOR ART
Heretofore, the method of photoluminescence has been employed for the study on deep level in the semiconductor crystal of a semiconductor wafer. The measurement of photoluminescence (PL) of semiconductor crystals is predominantly carried out at low temperatures such as the temperature of liquid nitrogen or that of liquid helium. In silicon crystals, only the low-temperature measurement has been performed effectively. In the low-temperature measurement, however, numerous deep luminescence levels and dopant levels originate in complicated mechanism of luminescence and defy analysis.
Recently, Tajima et al. developed a technique for enhancing the sensitivity of measurement of photoluminescence (hereafter abbreviated to PL) [M. Tajima, Jpn. J. Appl. Phys. 27, L1323-L1326 (1988)]. Owing to this technique, the observation of deep level PL spectrum at room temperature and the wafer mapping of PL intensity in a wafer which have been difficult to attain to date can be carried out with high sensitivity and with ease. The PL spectrum obtained by the room temperature PL measurement generally consists of one deep level luminescence having a PL intensity of I.sub.D at a wavelength of .lambda..sub.D and one band edge luminescence having a PL intensity of I.sub.B at a wavelength of .lambda..sub.B as illustrated in FIG. 2. In this case the mechanism of luminescence is conspicuously simple as compared with that which is involved in the low-temperature measurement. With respect to the deep level called EL2 in a semiinsulating (10.sup.7 -10.sup.9 .OMEGA.cm) GaAs crystal, therefore, it has been demonstrated that a strong correlation exists between the intensity I.sub.D of the deep level PL and the deep level concentration N.sub.D [M. Tajima, Appl. Phys. Lett. 53, 959-961(1988)]. In short, the relation of I.sub.D .alpha.N.sub.D is approximately satisfied in this case.
Incidentally, however, the intensity I.sub.D of deep level PL at room temperature is not always proportional to the deep level concentration N.sub.D. Generally, it has been known that this intensity I.sub.D is approximately proportional to the product of the deep level concentration N.sub.D multiplied by the carrier lifetime .tau. and is expressed as I.sub.D .alpha.N.sub.D .tau.. The result of the deep level (EL2) in GaAs reported by Tajima is considered to represent a special case in which .tau..apprxeq.constant in the expression of I.sub.D .alpha.N.sub.D .tau. is acceptable. Generally with respect to deep levels in semiconductors, however, there exist numerous cases in which the expression .tau..apprxeq. constant is not satisfied. With respect to the deep level PL (P-line) related to the thermal donor in the Si crystal and the deep level PL (D1-line) related to the precipitation of oxygen in the Si crystal, for example, it occurs not infrequently that the expression .tau..apprxeq. constant cannot be admitted as true and the proportional relation between I.sub.D and N.sub.D is not recognized.
When the spatial I.sub.D data of a semiconductor wafer are obtained by measurement and are subjected to mapping, the outcome of the mapping does not always correspond to the distribution of the deep level concentration N.sub.D.